Security firm Blackwater (now Xe) has been banned from Iraq
US security firm Blackwater is at the centre of another controversy - the CIA is alleged to have hired its contractors as part of a covert programme to kill top al-Qaeda figures.
A New York Times report says the company - which was renamed Xe in February - helped the CIA with planning, training and surveillance.
But the report says no militants were caught or captured.
Blackwater became known when its staff were used to guard US government personnel in Iraq from 2003.
In 2004, the burnt and mutilated bodies of its contractors were hung from a bridge across the river Euphrates in the Iraqi city of Falluja, to the cheers of an angry mob.
Four Blackwater men died during that ambush but, despite the deaths, the company's contractors remained among the most numerous in Iraq.
Based at a vast ranch complex in North Carolina, the firm described itself as "the most comprehensive professional military... company in the world."
In January 2008, the Iraqi government banned Blackwater over the involvement of its security guards in a 2007 incident which left 14 civilians dead.
Five former Blackwater guards have gone on trial in the US over the killings in Baghdad.
The firm's founder Erik Prince has stepped down as CEO
They have pleaded not guilty to killing 14 Iraqi civilians and wounding 18 others by gunfire and grenade.
The killings took place when Blackwater guards opened fire in Nisoor Square, Baghdad, while escorting an American diplomatic convoy.
The firm says its guards were acting in self-defence but witnesses and relatives of those killed maintain the shooting was unprovoked. Children were among the victims.
The killings strained Iraq-US relations and raised questions about the oversight of US contractors operating in war zones.
The Iraqi government pressed Washington to withdraw Blackwater from the country, but the security firm's contract was renewed in 2008.
But a new US-Iraqi security agreement gave Baghdad the authority to determine which western security companies operated in the country.
After Iraq decided to ban Blackwater, the US government said it would not renew the firm's task orders. It said another US company, Triple Canopy, was taking over the contract.
All Xe (Blackwater) staff will cease to operate in Iraq from the end of September 2009.
The shootings in Nisoor Square in September 2007 led to a debate about the role of private security companies operating in the country.
In 2004 a law drawn up by the Coalition Provisional Authority (CPA) - the now-defunct interim body set up by the US-led coalition in the wake of the fall of Saddam Hussein - had granted firms like Blackwater immunity from prosecution under Iraqi law.
Critics said such contractors, often elite soldiers recently retired from military special operations units, were little more than mercenaries, awarded lucrative packages to fight on demand in lawless areas.
Human rights campaigners said their uncertain legal status in Iraq - straddling international law, US regulations and Iraqi legislation - enabled them to act with virtual impunity.
Yet, despite the concerns about the privatisation of war and the lack of accountability of such fighters, some analysts argued that without the use of security firms, sustaining ongoing operations in Afghanistan and Iraq would be impossible.
Xe has defended its work in Iraq. A company spokeswoman has said: "When the US government initially asked for our help to assist with an immediate need to protect Americans in Iraq, we answered the call and performed well.
"We are proud of our success - no-one under our protection has been killed or even seriously wounded."
Blackwater was one of the first private security firms to work on the ground in Iraq following the US-led takeover, securing the contract to provide security for CPA head Paul Bremer.
Blackwater provided security in New Orleans after Hurricane Katrina
This was among the first of the many lucrative government contracts that reportedly made up 90% of the company's revenues.
The firm's base in North Carolina is reported to span several thousand acres and to include vast firing ranges, tactical exercise areas and an armoury.
Its founder, 40-year-old Erik Prince, is a former member of the US Navy's elite special operation force, the Navy Seals.
Originally concentrating on providing gun and firearms training for US government agencies, after the attacks of 11 September 2001, the company evolved.
Its website read: "We are a professional military, law enforcement, security, peacekeeping, and stability operations firm who provides turnkey solutions."
Mr Prince showed himself to be as able an entrepreneur as a soldier, ever quick to spot new business opportunities.
Blackwater provided security in New Orleans after the city was pounded by Hurricane Katrina.
And, according to a book on the firm written by Jeremy Scahill, in the aftermath of the 1999 Columbine school shootings in Colorado, the company built a fake school on the grounds where police teams could practise tracking down high-school killers to the sounds of screaming students.
Erik Prince stepped down as the CEO of company in March, but retained his title as chairman.
With the Iraq contract lost, Xe has been expanding into other lines of business.
It has a fleet of 76 aircraft that it has deployed to such hotspots as West Africa and Afghanistan, according to the Associated Press news agency.
It is also reportedly continuing to expand law enforcement training, with a renewed focus on international clients.